1. Field of the Invention
The present invention relates to navigation apparatus for searching for travel routes for a land vehicle, based on map information, and for transmitting information regarding the route(s) located by search to the driver of the vehicle. More particularly, the invention relates to route search apparatus with an improved scheme for searching for an optimum route leading to a destination, and to its associated navigation apparatus, as well as to a medium storing therein computer programs for execution of the improved search scheme.
2. Description of the Prior Art
Systems have recently been proposed for transmitting traffic information concerning roads, parking areas and the like between road information stations, such as for example, a vehicle information communications system (VICS) or automobile traffic information system (ATIS), and navigation apparatus built into land vehicles. More specifically, information as to roads and parking areas may be collected and organized at information acquisition/collection centers, including the Metropolitan Police Office, National Road Traffic Information Center, Parking Area Management Center, and the like. The collected information as to roads and the like is transmitted over the air from the VICS center by means of either radiation broadcasting--such as optical beacons, radio-wave beacons, FM multiplexing (frequency modulation multiplex scheme), or wireless telephones.
Transmitters for transmitting an optical beacon or radiowave beacon are installed at constant intervals along respective roads, either above each road or on the ground surface. When a land vehicle having a built-in receiver (light-receiving device or the like) for an optical beacon or radio beacon passes under or immediately above one of such transmitters, it receives VICS data. Also, in the case of FM multiplex radiation, VICS data may be received by commercially available radio modules capable of receiving FM broadcast programs. Further, in the case of wireless telephones, VICS data may be available through use of general-purpose public telephone sets such as handheld telephones.
The VICS data as transmitted by light or radio beacons may include information about road segments in a relatively narrow area of coverage near or around the beacon transmitter. Alternatively, the VICS data transmitted by either FM multiplex or wireless telephone may include information about roads in a relatively wide area.
In ATIS, information collected by the information acquisition agency, such as the Japanese Road Traffic Information Center, is transmitted via public telephone communication networks. In other words, unlike VICS, in ATIS, necessary road information is down-loaded by means of interactive communication. It is thus possible to receive items of information about roads or streets limited to a desired local area.
A navigation apparatus receiving road information in the form of VICS data, selectively changes a guidance route in accordance with the content of such road information. For example, when it is determined from the received road information that a street or road which is crowded or under construction is included within a previously set guidance route, the navigation apparatus conducts a new search to determine a new guidance route which detours the problem road. In other words, the guidance route may be changed or modified at any time when such a need is indicated by the up-to-date road information, thus enabling more comfortable travel toward either a destination or a temporary stopover location. Note here that the first searched guidance route is that determined by the initial search, i.e. at a point in time when a destination is set in the navigation apparatus or, alternatively, at the time when a stopover location is set therein. Where no roads or streets with traffic difficulties, such as traffic congestion, are detected during travel along a guidance route, travel guidance is performed in the usual manner along that guidance route.
However, it can happen in some cases that execution of such a guidance route change, in accordance with road information received from the road information station, such as VICS or the like, results in inconvenience or error. By way of example, where a land vehicle is traveling near or around a road-crossing point or "intersection" at which the vehicle should turn to the right or left to follow the prior searched guidance route, it might occur that a re-search is started to search for a guidance route in conformity with newly received road information. In this case, since a certain time is required for execution of such a re-search, during that time guidance routes may vanish from the display screen, while presentation of audible information is simultaneously interrupted. Further, if a guidance route change is made suddenly at a location near and in advance of the intersection at which the vehicle is to change its travel direction, such change might require a rapid change of vehicle operation, such as a lane change or the like, which change in vehicle operation might be difficult and even dangerous.